Progressing cavity pumps (PCP) were invented in the 1930s by Moineau as seen in U.S. Pat. Nos. 1,892,217 and 2,028,407.
A progressing cavity pump has a stator and a rotor. The stator typically comprises an elastomeric liner within a housing. The stator is open at both ends and has a multi-lobe helical passage extending through it. The rotor is normally of metal and has a helical exterior. Rotating the rotor causes fluid to pump through the stator. Progressing cavity pumps are used for a variety of purposes.
As a well pump, progressing cavity pumps may be driven by a downhole electrical motor or by a string of rods extending to a motor located at the surface. With a rod driven pump, normally the stator is suspended on a string of tubing, and the drive rods are located within the tubing. When installing a rod driven progressing cavity pump, the operator first secures the stator to the string of tubing and runs the tubing into the well to a desired depth. The operator then lowers the rotor through the tubing on the string of rods and into the stator.
To operate the pump at desired capacity, the rotor must be at the desired axial spacing within the stator and the rods must be in tension. If the lower end of the rotor is spaced above a lower end of the stator during operation, then a lower portion of the stator will not be in engagement with the rotor and the pumping capacity will suffer. The operator thus needs to know when the rotor has fully entered the stator during installation. The operator can calculate how much the rods will stretch due to the hydrostatic weight of the column of well fluid in the tubing. With the anticipated stretch distance known and with the rotor at a known initial position in the stator, the operator can pull the rods and rotor upward a distance slightly greater than the anticipated stretch, so that during operation, the rotor will move back downward to the desired axial position relative to the stator.
In the prior art, prior to running the tubing, the operator secures or welds a tag bar across the bottom of the stator. During installation, downward movement of the rods will stop when the lower end of the rotor contacts the tag bar at the bottom of the stator. Upon tagging the bar, the operator pulls the rod string back toward the surface by the calculated amount of rod stretch. During operation, as well fluid fills the tubing, the rod stretches, allowing the rotor to move back downward until in full engagement with the stator. If installed properly, once the rods have stretched fully, the lower end of the rotor will be spaced above the tag bar and the rods will be in tension.
While this method works well enough, tag bar creates an obstruction at the bottom of the pump. The obstruction prevents the operator from lowering tooling or instruments through and below the pump for logging, tagging fill, and other monitoring related purposes. Other problems with this approach are the obstruction to flow during operation, and the tendency of sand and well debris to accumulate around the tag bar and clog the intake.
U.S. Pat. No. 7,201,222 teaches of a tag method in which the tag location is an interference shoulder above the pump. The tag shoulder is located above the stator in a reduced diameter collar connected to the tubing, while the rotor tag is connected to the rod string above the rotor. When the rotor is lowered down and reaches its appropriate location relative to the stator, the stop on the rotor rod string interferes with the reduced diameter collar located above the stator in the tubing string, preventing the rotor from progressing further into the stator. While some of the above issues were overcome with this method, there was still the issue of proper placement of the tag bar with respect to the stator. To avoid the eccentric rotation of the rotor, proper distance had to be placed between the tag area and the top of the stator. As the tag location on the collar has to match up directly with the tag location on the rotor rod string, long, precision equipment would be required, as well as specialized equipment to prevent the stop on the rotor rod string from damaging the tubing as the rod string rotated. In addition, this method would present more flow obstruction problems, now moved from below the pump to above the pump.
Similarly, U.S. Publication 2009/0136371 suggests a method that lowers the tag surface to the very top of the stator, by shaping the pass through hole in the tag collar located in the tubing string above the stator or integral with the stator, in such a way that the rotor eccentric motion would not cause the rotor to contact the through hole. More simply, the opening is shaped like the stator helical cavity, so as the collar is placed directly above the stator and timed correctly, the rotor should operate freely in the collar. The rotor head would then locate on what would be the minor diameter of the collar through hole. To avoid damage from heat if welding was used to secure the tag bar above the stator, there still needed to be a substantial spacing between the stator top and the tag bar. If connections that were threaded were used instead there were still placement issues could exist. A threaded connection was difficult to properly torque while still winding up with the needed alignment of the oblong openings. If the thread had to be backed up after being torqued to align the stator and collar openings then the torque for the connection was reduced which risked the connection getting subsequently undone while the pump was in service.
Also relevant to issues of rotor placement are U.S. Pat. Nos. 5,209,294 and 5,725,053.
The present invention addresses the issues with bottom tag bar systems of the past by forming the bottom tag where it can stop rotor movement for alignment purposes but at the same time provide a passage through it for tubing access for cleanup of the wellbore when the rotor is pulled. The passage in the tag device can be made small enough to refuse the rotor minor diameter or larger so that the major diameter provides the travel stop for the rotor generally after advancing the length of half the rotor pitch. Those skilled in the art will more readily appreciate further aspects of the invention from a review of the detailed description of the preferred embodiment and the associated drawings while understand that the appended claims delimit the full scope of the invention.